JP7358157B2 - Complex oxide catalyst, porous composite, and method for producing complex oxide catalyst - Google Patents

Description

The present invention relates to a composite oxide catalyst, a method for producing the same, and a porous composite containing the composite oxide catalyst.

Vehicles equipped with a diesel engine or a gasoline engine are equipped with a filter that collects and burns (that is, oxidizes) particulate matter (PM) such as soot in exhaust gas. Patent Document 1 proposes, as a particulate matter combustion catalyst supported on an exhaust gas purification filter, a catalyst having a plurality of laminated alumina layers and an Ag alloy dispersed between them. The Ag alloy is at least one selected from the group of AgZr alloy, AgPt alloy, and AgPd alloy.

Patent Document 2 proposes an exhaust gas purification catalyst for oxidizing and removing particulate matter contained in exhaust gas from a lean burn engine. The exhaust gas purification catalyst includes a first metal oxide selected from the group consisting of praseodymium oxide, terbium oxide, and combinations thereof, and a group consisting of lanthanum oxide, neodymium oxide, yttrium oxide, and combinations thereof. The second metal oxide is selected from the following.

The exhaust gas purification catalyst proposed in Patent Document 3 includes a first metal oxide selected from the group consisting of praseodymium oxide, terbium oxide, and combinations thereof, a second metal oxide that is neodymium oxide, a third metal oxide that is zirconia or a combination of zirconia and ceria, and selected from the group consisting of lanthanum oxide, yttrium oxide, barium oxide, calcium oxide, strontium oxide, silicon oxide, and combinations thereof. The fourth metal oxide contains a fourth metal oxide.

JP2016-36782A Japanese Patent Application Publication No. 2014-124631 Japanese Patent Application Publication No. 2014-200771

By the way, in the particulate matter combustion catalyst of Patent Document 1, the use of platinum group elements increases the manufacturing cost of the particulate matter combustion catalyst. In addition, in the particulate matter combustion catalyst, the PM combustion rate decreases significantly in high-temperature durability tests. The exhaust gas purification catalysts of Patent Document 2 and Patent Document 3 further contain platinum group elements such as platinum and iridium, which increases manufacturing costs.

Currently, there is a need for a catalyst that can burn (i.e., oxidize) objects such as particulate matter in exhaust gas at low temperatures without using expensive materials such as platinum group elements.

The present invention was made in view of the above problems, and an object of the present invention is to lower the oxidation initiation temperature of a target object.

The present invention is directed to a composite oxide catalyst that promotes combustion of particulate matter in exhaust gas. A composite oxide catalyst according to a preferred embodiment of the present invention contains, as metals, cerium as a first metal, lanthanum as a second metal, and a third metal. The third metal is manganese or praseodymium . The content of cerium in the metal contained is 10 mol% or more and 93 mol% or less. The content of lanthanum in the contained metal is 3.5 mol% or more and 45 mol% or less. When the third metal is manganese, the content of the third metal in the contained metal is 3.5 mol% or more and 45 mol% or less. When the third metal is praseodymium, the content of the third metal in the contained metal is 10 mol%. The content rate of lanthanum in the included metal is equal to the content rate of the third metal in the included metal. The content of metals other than cerium, lanthanum, and the third metal in the contained metal is 1 mol% or less. The composite oxide catalyst includes particles having cavities inside in a SEM image at a magnification of 5000 times . The diameter of the cavity is 0.2 μm or more and 5 μm or less.

Preferably, the content of cerium in the metal contained is 80 mol% or more and 93 mol% or less. The content of lanthanum in the contained metal is 3.5 mol% or more and 10 mol% or less. When the third metal is manganese, the content of the third metal in the contained metal is 3.5 mol% or more and 10 mol% or less.

The invention is also directed to porous composites. A porous composite according to one preferred embodiment includes a porous base material and a porous collection layer formed on the base material to collect particulate matter in exhaust gas . The collection layer includes the above-mentioned composite oxide catalyst.

Preferably, the base material has a honeycomb structure in which the inside is partitioned into a plurality of cells by partition walls. The inner surfaces of at least some of the plurality of cells are covered with the collection layer.

More preferably, the porous composite is a filter that collects particulate matter in exhaust gas discharged from a gasoline engine or a diesel engine.

The present invention is also directed to a method for producing a composite oxide catalyst that promotes combustion of particulate matter in exhaust gas. A method for producing a composite oxide catalyst according to a preferred embodiment includes the steps of: preparing a mixed aqueous solution by mixing a nitride of a raw metal and citric acid; and heating the mixed aqueous solution to obtain a precursor. and a step of calcining the precursor to obtain a composite oxide catalyst. The raw material metal includes cerium as a first metal, lanthanum as a second metal, and a third metal. The third metal is manganese or praseodymium. The content of cerium in the raw material metal is 10 mol% or more and 93 mol% or less. The content of lanthanum in the raw material metal is 3.5 mol% or more and 45 mol% or less. When the third metal is manganese, the content of the third metal in the raw material metal is 3.5 mol% or more and 45 mol% or less. When the third metal is praseodymium, the content of the third metal in the raw material metal is 10 mol%. The content of lanthanum in the raw metal is equal to the content of the third metal in the raw metal . The content of metals other than cerium, lanthanum, and the third metal in the raw material metal is 1 mol% or less. The composite oxide catalyst includes particles having cavities inside in a SEM image at a magnification of 5000 times. The diameter of the cavity is 0.2 μm or more and 5 μm or less.

In the present invention, the oxidation start temperature of the target object can be lowered.

FIG. 1 is a plan view of a porous composite according to one embodiment. FIG. 2 is a cross-sectional view of a porous composite. FIG. 3 is a diagram showing the flow of manufacturing a porous composite. FIG. 2 is a diagram showing the flow of manufacturing a composite oxide catalyst. 1 is a SEM image of the composite oxide catalyst of Example 1. 3 is a SEM image of the composite oxide catalyst of Example 11.

FIG. 1 is a simplified plan view showing a porous composite 1 according to one embodiment of the present invention. The porous composite 1 is a cylindrical member that is long in one direction, and FIG. 1 shows one end surface of the porous composite 1 in the longitudinal direction. FIG. 2 is a cross-sectional view showing the porous composite 1. As shown in FIG. FIG. 2 shows a part of the cross section along the longitudinal direction. The porous composite body 1 is used, for example, as a gasoline particulate filter (GPF) that collects particulate matter such as soot in exhaust gas discharged from a gasoline engine such as an automobile.

The porous composite 1 includes a porous base material 2 and a porous collection layer 3. In the example shown in FIGS. 1 and 2, the base material 2 is a member having a honeycomb structure. The base material 2 includes a cylindrical outer wall 21 and a partition wall 22. The cylindrical outer wall 21 is a cylindrical portion extending in the longitudinal direction (that is, the left-right direction in FIG. 2). The cross-sectional shape of the cylindrical outer wall 21 perpendicular to the longitudinal direction is, for example, approximately circular. The cross-sectional shape may be other shapes such as a polygon.

The partition wall 22 is a lattice-shaped portion that is provided inside the cylindrical outer wall 21 and partitions the inside into a plurality of cells 23 . Each of the plurality of cells 23 is a space extending in the longitudinal direction. The cross-sectional shape of each cell 23 perpendicular to the longitudinal direction is, for example, approximately square. The cross-sectional shape may be polygonal or other shapes such as circular. The plurality of cells 23 basically have the same cross-sectional shape. The plurality of cells 23 may include cells 23 having different cross-sectional shapes. The base material 2 is a cell structure whose interior is partitioned into a plurality of cells 23 by partition walls 22 .

The cylindrical outer wall 21 and the partition wall 22 are each porous portions. The cylindrical outer wall 21 and the partition wall 22 are made of ceramic, for example. _The main material of the cylindrical outer wall 21 and the partition wall 22 is preferably cordierite (2MgO.2Al ₂ O 3.5SiO ₂ ). The material of the cylindrical outer wall 21 and the partition wall 22 may be ceramic other than cordierite, or may be a material other than ceramic.

The length of the cylindrical outer wall 21 in the longitudinal direction is, for example, 50 mm to 300 mm. The outer diameter of the cylindrical outer wall 21 is, for example, 50 mm to 300 mm. The thickness of the cylindrical outer wall 21 is, for example, 30 μm (micrometers) or more, and preferably 50 μm or more. The thickness of the cylindrical outer wall 21 is, for example, 1000 μm or less, preferably 500 μm or less, and more preferably 350 μm or less.

The length of the partition wall 22 in the longitudinal direction is approximately the same as that of the cylindrical outer wall 21. The thickness of the partition wall 22 is, for example, 30 μm or more, preferably 50 μm or more. The thickness of the partition wall 22 is, for example, 1000 μm or less, preferably 500 μm or less, and more preferably 350 μm or less. The porosity of the partition wall 22 is, for example, 20% or more, preferably 30% or more. The porosity of the partition wall 22 is, for example, 80% or less, preferably 70% or less. The porosity can be measured, for example, by the Archimedes method using pure water as a medium. The average pore diameter of the partition walls 22 is, for example, 5 μm or more, preferably 8 μm or more. The average pore diameter of the partition walls 22 is, for example, 30 μm or less, preferably 25 μm or less. The average pore diameter is measured, for example, by mercury porosimetry (according to JIS R1655).

The cell density (i.e., the number of cells 23 per unit area in a cross section perpendicular to the longitudinal direction) of the base material 2 is, for example, 10 cells/cm ² (square centimeter) or more, preferably 20 cells/cm ² or more. , more preferably 30 cells/cm ² or more. The cell density is, for example, 200 cells/cm ² or less, preferably 150 cells/cm ² or less. In FIG. 1, the size of the cells 23 is drawn larger than the actual size, and the number of cells 23 is drawn smaller than the actual size. The size, number, etc. of the cells 23 may be changed in various ways.

When the porous composite 1 is used as a GPF, one end of the porous composite 1 in the longitudinal direction (i.e., the left side in FIG. 2) is used as an inlet, and the other end is used as an outlet. Gases such as exhaust gas flow through. Further, among the plurality of cells 23 of the porous composite 1, a plugging portion 24 is provided at the end on the inlet side in some of the plurality of cells 23, and in the remaining plurality of cells 23, the plugging portion 24 is provided at the end on the exit side. A plugging portion 24 is provided at the end.

FIG. 1 depicts the inlet side of the porous composite 1. Further, in FIG. 1, the plugging portion 24 on the entrance side is shaded with parallel diagonal lines in order to facilitate understanding of the figure. In the example shown in FIG. 1, a cell 23 is provided with a plugged portion 24 on the inlet side, and a cell 23 is not provided with a plugged portion 24 on the inlet side (i.e., a cell 23 is provided with a plugged portion 24 on the outlet side). The provided cells 23) are arranged alternately in the vertical and horizontal directions in FIG.

In the following description, the cell 23 provided with the plugging section 24 on the outlet side will be referred to as a "first cell 231", and the cell 23 provided with the plugging section 24 on the inlet side will be referred to as a "second cell 232". It is called. Furthermore, when there is no need to distinguish between the first cell 231 and the second cell 232, they are collectively referred to as the "cell 23" as described above. In the plurality of cells 23 of the porous composite 1, a plurality of first cells 231 that are sealed at one end in the longitudinal direction and a plurality of second cells 232 that are sealed at the other end in the longitudinal direction are arranged alternately. has been done.

The collection layer 3 is formed in the form of a film on the surface of the base material 2 . In the example shown in FIG. 2, the collection layer 3 is provided in a plurality of first cells 231 in which plugging portions 24 are provided on the outlet side, and the trapping layer 3 is provided on the inner surface of the plurality of first cells 231 (i.e., a partition wall). 22). In FIG. 2, the collection layer 3 is shown by a thick line. The collection layer 3 also covers the inner surface of the plugging section 24 on the outlet side within the plurality of first cells 231 . On the other hand, the collection layer 3 is not present in the plurality of second cells 232 in which the plugging portions 24 are provided on the inlet side. In other words, the inner surfaces of the plurality of second cells 232 are not covered with the collection layer 3 and are exposed.

The collection layer 3 in the plurality of first cells 231 is mainly formed of ceramic, for example. In this case, the trapping layer 3 preferably contains at least one of silicon carbide, cordierite, mullite, aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, iron oxide, and cerium oxide as a main material. Note that the collection layer 3 may be formed using another ceramic as a main material, or may be formed using a material other than ceramics (for example, a composite oxide catalyst described later) as a main material.

The average pore diameter of the collection layer 3 is preferably 0.1 μm or more and 20 μm or less. The average pore diameter is more preferably 4.1 μm or more and 20 μm or less, and still more preferably 4.1 μm or more and 6 μm or less. The porosity of the collection layer 3 is preferably 50% or more and 90% or less. The porosity is more preferably 70% or more and 78% or less. The average particle size of the aggregate constituting the collection layer 3 is preferably 0.1 μm or more and 5 μm or less. The average particle size is more preferably 0.4 μm or more and 5 μm or less.

The average pore diameter and porosity of the collection layer 3 and the average particle size of the aggregate can be determined by image analysis of a SEM (scanning electron microscope) image of a polished cross section of the porous composite 1. The image analysis is performed using, for example, image analysis software "Image-Pro ver. 9.3.2" manufactured by Nippon Roper Co., Ltd.

The overall average film thickness (hereinafter also referred to as "overall average film thickness") of the collection layer 3 is preferably 10 μm or more and 40 μm or less. The overall average film thickness is more preferably 30 μm or more and 40 μm or less. Moreover, the average film thickness of the collection layer 3 at the end on the exit side (hereinafter also referred to as "exit side average film thickness") is 20 μm or more and 50 μm or less. The outlet side average film thickness is more preferably 35 μm or more and 50 μm or less. Preferably, the outlet side average film thickness is larger than the overall average film thickness. The average film thickness of the collection layer 3 can be measured using, for example, a 3D shape measuring device.

In the porous composite 1 shown in FIGS. 1 and 2, as shown by arrow A1 in FIG. It flows into the first cell 231 from the inlet, passes through the collection layer 3 and the partition 22 from the first cell 231, and moves to the second cell 232 whose outlet side is not sealed. At this time, particulate matter in the gas is efficiently collected in the collection layer 3.

The collection layer 3 further includes a composite oxide catalyst that promotes combustion (i.e., oxidation removal) of the collected particulate matter. The composite oxide catalyst is preferably metal oxide particles (that is, fine particles made of metal oxide) fixed to the main material of the collection layer 3 (eg, the above-mentioned ceramic). Note that the composite oxide catalyst does not necessarily need to be fixed to the main material of the collection layer 3 in the form of particles, and may be solidly dissolved in the main material, for example. Further, as described above, the main material of the collection layer 3 may be formed using the composite oxide catalyst as the main material. In this case, the main material may contain, for example, the above-mentioned ceramic in addition to the composite oxide catalyst.

The metals contained in the composite oxide catalyst (hereinafter referred to as "containing metals") are cerium (Ce) which is a first metal, lanthanum (La) which is a second metal, and a third metal. In other words, the composite oxide catalyst contains Ce, La, and a third metal as contained metals. The third metal is a transition metal or a rare earth metal other than Ce and La. When the third metal is a transition metal, manganese (Mn) is preferably utilized as the third metal. When the third metal is a rare earth metal, praseodymium (Pr) is preferably utilized as the third metal. The third metal may be a transition metal other than Mn (for example, cobalt (Co), etc.) or a rare earth metal other than Pr (excluding Ce and La).

The content of Ce in the metal contained in the composite oxide catalyst (hereinafter also simply referred to as "contained metal") is 5 mol% or more and 95 mol% or less. The content of La in the metal contained is 2 mol% or more and 93 mol% or less. The content rate of the third metal in the contained metal is 2 mol% or more and 93 mol% or less.

Preferably, the content of Ce in the contained metal is 50 mol% or more and 95 mol% or less, the La content is 2 mol% or more and 48 mol% or less, and the content of the third metal is 2 mol% or more and 48 mol% or less. It is. More preferably, the third metal is Mn or Pr, and the content rate of the third metal in the included metal is substantially equal to the content rate of La in the included metal.

Preferably, the total content of Ce, La and third metal in the included metal is substantially 100 mol%. Note that the state in which the total content is substantially 100 mol% refers to the state in which the content of impurity metals (that is, metals other than Ce, La, and the third metal) in the composite oxide catalyst is 1 mol% or less. means.

Next, an example of a method for manufacturing the porous composite 1 will be described with reference to FIG. 3. When the porous composite 1 is manufactured, first, the outer surface of the cylindrical outer wall 21 of the base material 2 is covered with a liquid-impermeable sheet member. For example, a liquid-impermeable film is wrapped around substantially the entire outer surface of the cylindrical outer wall 21 .

Subsequently, a raw material slurry for forming the collection layer 3 is prepared (step S11). The raw material slurry is prepared by adding and mixing particles that are the raw material for the collection layer 3 (hereinafter referred to as "collection layer particles"), pore-forming agent particles, flocculant, etc. . The collection layer particles include, for example, particles of silicon carbide (SiC) or cerium oxide (CeO ₂ ), which are the main raw materials of the above-mentioned collection layer 3, and particles of a composite oxide catalyst. Further, as described above, when the main material of the trapping layer 3 is mainly formed of a composite oxide catalyst, the trapping layer particles are mainly particles of the composite oxide catalyst.

The raw material slurry contains particles in which collection layer particles, pore-forming agent particles, and the like are aggregated (hereinafter referred to as "agglomerated particles"). When the raw material slurry is prepared, the type, amount, etc. of the flocculant are determined so that the particle size of the aggregated particles is larger than the average pore size of the base material 2. This prevents or suppresses the agglomerated particles from entering the pores of the base material 2 in step S12, which will be described later. The viscosity of the raw material slurry is, for example, 2 mPa·s to 30 mPa·s.

Next, among the plurality of cells 23 of the base material 2, the entrances of the plurality of first cells 231 (i.e., the plugging portions) are The raw material slurry is supplied from the end (the end where 24 is not provided) (step S12). The water in the raw material slurry passes through the partition wall 22 of the base material 2 and moves to the adjacent second cell 232, and flows from the end of the second cell 232 where the plugging portion 24 is not provided to the base material 2. It flows out to the outside of the material 2. The aggregated particles in the raw material slurry do not pass through the partition wall 22 and adhere to the inner surface of the first cell 231 to which the raw material slurry is supplied. As a result, an intermediate body in which the aggregated particles are substantially evenly adhered to the inner surface of the first cell 231 of the base material 2 is formed.

When the supply of a predetermined amount of raw material slurry is completed, the intermediate from which water has flowed out is dried (step S13). For example, the intermediate is dried at room temperature for 12 hours and then further dried by heating at 80° C. for 12 hours. Thereafter, by firing the intermediate, the collection layer particles among the many aggregated particles adhering to the base material 2 are combined and spread over the surface of the base material 2, forming a porous collection layer 3. (Step S14). In the firing step, pore-forming agent particles contained in the collection layer 3 are burned and removed, thereby forming pores in the collection layer 3. The firing temperature and firing time in step S14 are, for example, 1000° C. and 2 hours, respectively.

Next, an example of a method for manufacturing the composite oxide catalyst contained in the raw material slurry in step S11 described above will be described with reference to FIG. 4. The manufacturing method shown in FIG. 4 is a manufacturing method called the citric acid method. When a composite oxide catalyst is manufactured, first, the weighed raw metal nitride is dissolved in ion-exchanged water together with the additive citric acid, so that the raw metal nitride and citric acid are dissolved. A mixed aqueous solution is prepared (step S21). The raw metal nitrides are cerium nitrate (Ce(NO ₃ ) ₃ ), lanthanum nitrate (La(NO ₃ ) ₃ ), and third metal nitride (that is, third metal nitrate).

The ratio of the total number of moles of nitride of the raw metal to the number of moles of citric acid in step S21 is, for example, 1:1. The preparation in step S21 is performed in a heated state, for example, at 80°C. In addition, in the weighing of the nitride of the raw material metal in step S21, the contents of Ce, La, and the third metal in the metal contained in the composite oxide catalyst are determined to be the desired values. The metal content is determined. Note that the contents of Ce, La, and the third metal in the raw metal are substantially equal to the contents of Ce, La, and the third metal in the metal contained in the composite oxide catalyst, respectively.

Subsequently, the above mixed aqueous solution is heated to obtain a composite oxide catalyst precursor (step S22). In step S22, for example, the mixed aqueous solution is concentrated by heating at 100° C. to 120° C., and further heated at 150° C. to 300° C., thereby obtaining the powder of the precursor.

Thereafter, the precursor is fired to obtain a composite oxide catalyst (step S23). In step S23, for example, the precursor powder is fired at 400° C. for 4 hours in the atmosphere, and the fired material is ground in an agate mortar. Further, the pulverized substance is calcined at 1000° C. for 2 hours in the atmosphere, and the calcinated substance is pulverized again in an agate mortar to obtain a composite oxide catalyst.

The composite oxide catalyst may be produced by a method other than the above-mentioned citric acid method. For example, the composite oxide catalyst may be manufactured by an impregnation support method. In this case, first, weighed lanthanum nitrate and a nitride of the third metal are dissolved in ion exchange water to prepare an aqueous solution. Further, the weighed cerium oxide is suspended in ion exchange water different from the aqueous solution to prepare a suspension. Subsequently, a precursor of a composite oxide catalyst is obtained by heating the liquid in which the aqueous solution is added dropwise to the suspension. Specifically, for example, the suspension containing the cerium oxide powder is heated at 100° C. to 120° C. to dryness, and then further heated at 150° C. to 300° C. to form the precursor powder. is obtained. Thereafter, the precursor is fired to obtain a composite oxide catalyst. Specifically, for example, the powder of the above precursor is fired at 400° C. for 4 hours in the atmosphere, and the fired material is ground in an agate mortar. Further, the pulverized substance is calcined at 1000° C. for 2 hours in the atmosphere, and the calcinated substance is pulverized again in an agate mortar to obtain a composite oxide catalyst.

Moreover, the composite oxide catalyst may be manufactured by a complex polymerization method. In this case, first, the weighed raw material metal compound is dissolved in glycol together with the additive oxycarboxylic acid to create a glycol solution (hereinafter also referred to as a "mixed solution") in which the raw material metal compound and oxycarboxylic acid are mixed. ) is prepared. The raw metal compound is, for example, an alkoxide, halide, carbonate, sulfate, carboxylate, hydroxide, nitride, etc. of the raw metal. Examples of the oxycarboxylic acids include citric acid, tartaric acid, malic acid, tartronic acid, glyceric acid, oxybutyric acid, hydroacrylic acid, lactic acid, and glycolic acid. Examples of the glycol include ethylene glycol, propylene glycol, trimethylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, and the like. For example, cerium nitrate, lanthanum nitrate, and a nitride of a third metal are used as raw metal compounds, and citric acid and propylene glycol are used as oxycarboxylic acids and glycols. The ratio of the total number of moles of the raw metal compound to the number of moles of oxycarboxylic acid is, for example, 1:10. The mixed solution is prepared, for example, while being heated to 80°C.

Subsequently, the above-mentioned mixed solution is heated to obtain a composite oxide catalyst precursor. Specifically, for example, the mixed solution is gelled by heating at 120° C. to 150° C., and then further heated at 300° C. to 400° C. to obtain the powder of the precursor. Thereafter, the precursor is fired to obtain a composite oxide catalyst. Specifically, for example, the powder of the above precursor is fired at 400° C. for 4 hours in the atmosphere, and the fired material is ground in an agate mortar. Further, the pulverized substance is calcined at 1000° C. for 2 hours in the atmosphere, and the calcinated substance is pulverized again in an agate mortar to obtain a composite oxide catalyst.

Next, the relationship between the composite oxide catalyst and the soot oxidation initiation temperature will be explained with reference to Tables 1 and 2.

In the composite oxide catalysts of Examples 1 to 11, the content of Ce in the metals contained was 10 mol% to 93 mol%, the content of La was 3.5 mol% to 45 mol%, and the content of the third metal was , 3.5 mol% to 45 mol%. In Examples 1-7 and Examples 10-11, the third metal is Mn. In Example 8, the third metal is Pr, and in Example 9, the third metal is Co.

In Examples 1 to 9, composite oxide catalysts were produced by the citric acid method described above. FIG. 5 is a SEM image obtained by photographing the composite oxide catalyst of Example 1 using a SEM at a magnification of 5000 times. The composite oxide catalyst of Example 1 is bulky and includes sparse particles having relatively large cavities inside, as surrounded by a circle labeled 91 in FIG. In Table 2 , the structure of the composite oxide catalyst of Example 1 is described as "sparse". In Examples 2 to 9 as well, the structure of the composite oxide catalyst is "sparse" as in Example 1. The composite oxide catalysts of Examples 1 to 9 contain, for example, sparse particles having a large number of cavities with diameters of 0.2 μm or more and 5 μm or less.

The diameter of the cavity is determined by the following method. First, the surface of the composite oxide catalyst is photographed using a SEM at a magnification of 5000 times to obtain a SEM image as shown in FIG. Subsequently, the SEM image is analyzed using, for example, the above-mentioned image analysis software "Image-Pro ver. 9.3.2" to determine the diameter of the cavity included in the particles of the composite oxide catalyst. The diameter of the cavity is, for example, the Feret diameter (JIS Z 8827-1) of the cavity (that is, the dark part existing in the particle that is the bright part) in the above SEM image. The Feret diameter is the diameter of the two parallel straight lines when the cavity is sandwiched between two parallel straight lines pointing in a predetermined direction (for example, the left-right direction) and the two straight lines are circumscribed to the cavity in the SEM image. spacing (ie, the distance between the two straight lines in the direction perpendicular to the two straight lines).

In Example 10, the composite oxide catalyst was manufactured by the above-mentioned impregnation support method, and in Example 11, the composite oxide catalyst was manufactured by the above-described complex polymerization method. FIG. 6 is a SEM image obtained by photographing the composite oxide catalyst of Example 11 using an SEM at a magnification of 5000 times. The structure of the composite oxide catalyst of Example 11 is "dense", unlike the "loose" structure of Example 1. Specifically, in the composite oxide catalyst of Example 11, in the SEM image at a magnification of 5000 times, there are no cavities of a size that can be seen in the particles, or there are cavities in the particles. Even if confirmed, the diameter of the cavity is less than 0.2 μm. In this case, "x" is written in the cavity diameter column of Table 2. In Example 10 as well, the structure of the composite oxide catalyst is "dense" as in Example 11.

In the oxide catalyst of Comparative Example 1, the content of Ce in the contained metals is 100 mol%. That is, the oxide catalyst of Comparative Example 1 contains only one type of metal. On the other hand, the oxide catalysts of Comparative Examples 2 to 4 contain only two types of metals. In the oxide catalyst of Comparative Example 2, the content of Ce in the metals contained was 80 mol%, and the content of La was 20 mol%. In the oxide catalyst of Comparative Example 3, the content of Ce in the metals contained was 80 mol%, and the content of Mn, which was the third metal, was 20 mol%. In the oxide catalyst of Comparative Example 4, the content of La in the metals contained was 50 mol%, and the content of Mn, which was the third metal, was 50 mol%. In Comparative Examples 1 to 4, the oxide catalysts were produced by the citric acid method described above. The structures of the oxide catalysts of Comparative Examples 1 to 4 are "sparse" as in Examples 1 to 9.

Table 1 shows that soot obtained by burning gasoline (so-called gasoline soot, hereinafter simply referred to as "soot") was mixed with the composite oxide catalyst of Examples 1 to 11 or the composite oxide catalyst of Comparative Examples 1 to 4. This shows the oxidation start temperature of soot when mixed with an oxide catalyst and heated. Comparative Example 5 shows the soot oxidation initiation temperature when no catalyst is used.

The soot oxidation initiation temperature was determined as follows. In Examples 1 to 11, first, composite oxide catalyst powder and soot were mixed at a weight ratio of 95:5 to obtain a mixed sample of composite oxide catalyst and soot. In the mixed sample, the soot and the composite oxide catalyst are in a loose contact (LC) state similar to the contact state between the particulate matter collected by the collection layer 3 and the collection layer 3 . Subsequently, the mixed sample was measured using a TPD-MS (thermal gas analysis) device. Specifically, 0.04 g of the _mixed sample was subjected to a predetermined heating program ( e.g. The temperature was increased to 700°C by heating at a heating rate of 20°C/min. Then, changes in the concentration of gas generated from the mixed sample as it was heated were measured, and changes in the amount of carbon monoxide (CO) produced by combustion (i.e., oxidation) of soot were obtained. Thereafter, the temperature at which the cumulative amount of CO produced reached 20% of the total amount of CO produced was defined as the temperature at which soot oxidation occurred (that is, the oxidation start temperature). The lower the oxidation start temperature, the higher the catalytic ability of the composite oxide catalyst.

In Comparative Examples 1 to 4, the oxidation start temperature was measured in the same manner as above, except that mixed samples were prepared using the oxide catalyst of the comparative example instead of the composite oxide catalyst described above. In Comparative Example 5, the oxidation start temperature was measured in the same manner as above, except that soot with no catalyst mixed therein was used instead of the mixed sample described above.

As shown in Comparative Example 5, the oxidation initiation temperature in the case of only soot is as high as 450°C. In addition, the oxidation start temperatures of Comparative Examples 1 to 4 are also relatively high at 364°C to 389°C. On the other hand, the oxidation start temperatures of Examples 1 to 11 are 313° C. to 355° C., which are lower than those of Comparative Examples 1 to 5.

In Examples 1 to 11, the oxidation start temperature of Examples 1 to 9 (manufacturing method: citric acid method, structure of composite oxide catalyst: sparse) was 313°C to 345°C, and that of Examples 10 to 11 (manufacturing method : Impregnation support method or complex polymerization method, complex oxide catalyst structure: lower than the oxidation initiation temperature of 351°C to 355°C. Further, the oxidation start temperature of Example 1 (third metal: Mn, 10 mol%) and Example 8 (third metal: Pr, 10 mol%) is 320°C to 322°C, and Example 9 (third metal: Co, 10 mol%) is lower than the oxidation initiation temperature of 345°C.

The oxidation start temperature of Example 1 (La content = Mn content, La content + Mn content = 20 mol%) was 320°C, and in Examples 6 to 7 (La content ≠ Mn content, La content + Mn content) content = 20 mol%), which is lower than the oxidation start temperature of 340°C. The oxidation start temperature of Examples 1 to 3 (Ce content: 52 mol% to 93 mol%) is 313°C to 323°C, and the oxidation start temperature of Examples 4 to 5 (Ce content: 10 mol% to 30 mol%) is 342°C to 342°C. Lower than 345°C.

As explained above, the composite oxide catalyst contains Ce, which is the first metal, La, which is the second metal, and the third metal, as contained metals. The third metal is a transition metal or a rare earth metal other than Ce and La. The content of Ce in the contained metal is 5 mol% or more and 95 mol% or less. The content of La in the contained metal is 2 mol% or more and 93 mol% or less. The content rate of the third metal in the contained metal is 2 mol% or more and 93 mol% or less. According to the composite oxide catalyst, as described above, the oxidation start temperature of objects such as particulate matter can be lowered. Moreover, since the composite oxide catalyst does not substantially contain expensive metals such as platinum group elements, the manufacturing cost of the composite oxide catalyst can be reduced.

As described above, in the composite oxide catalyst, the content of Ce in the metal contained is preferably 50 mol% or more and 95 mol% or less. The content of La in the contained metal is preferably 2 mol% or more and 48 mol% or less. The content of the third metal in the contained metal is preferably 2 mol% or more and 48 mol% or less. Thereby, the oxidation start temperature of the target such as particulate matter can be further lowered.

More preferably, the third metal is Mn or Pr, and the content rate of La in the included metal is equal to the content rate of the third metal in the included metal. Thereby, the oxidation start temperature of the target such as particulate matter can be further lowered.

As described above, it is preferable that the composite oxide catalyst includes particles having cavities inside in a SEM image at a magnification of 5000 times. In the composite oxide catalyst, the cavity is utilized for contact between the target object such as particulate matter and the composite oxide catalyst, thereby making it possible to increase the contact area between the target object and the composite oxide catalyst. As a result, the oxidation start temperature of the object can be further lowered.

More preferably, the diameter of the cavity is 0.2 μm or more and 5 μm or less. This facilitates the entry of objects such as particulate matter into the cavity, thereby promoting contact between the objects and the composite oxide catalyst. As a result, the oxidation start temperature of the object can be lowered even further.

As described above, the porous composite 1 includes the porous base material 2 and the porous collection layer 3 formed on the base material 2. The collection layer 3 contains the above-mentioned composite oxide catalyst. Thereby, in the porous composite 1, the particulate matter collected by the collection layer 3 can be oxidized (ie, burned) and removed at a low temperature. As a result, the heat resistance required of the porous composite 1 can be relaxed, and the degree of freedom in designing the porous composite 1 can be improved.

As described above, the base material 2 has a honeycomb structure in which the inside is partitioned into a plurality of cells 23 by partition walls 22, and the inner surface of at least some of the cells 23 is covered with the collection layer 3. It is preferable to be coated with. According to the porous composite body 1 having the structure, it is possible to achieve both suitable collection of particulate matter and suppression of pressure loss. Further, as described above, according to the porous composite 1, the oxidation of the collected particulate matter can be promoted, and the oxidation start temperature of the particulate matter can be lowered. Therefore, the porous composite 1 is particularly suitable for GPF, which collects particulate matter in exhaust gas discharged from gasoline engines.

The method for producing the above-mentioned composite oxide catalyst includes a step of preparing a mixed aqueous solution by mixing the nitride of the raw metal and citric acid, a step of heating the mixed aqueous solution to obtain a precursor, and a step of firing the precursor. and obtaining a composite oxide catalyst. The raw material metal includes Ce, which is a first metal, La, which is a second metal, and a third metal. The third metal is a transition metal or a rare earth metal other than Ce and La. The content of Ce in the raw metal is 5 mol% or more and 95 mol% or less. The content of La in the raw metal is 2 mol% or more and 93 mol% or less. The content of the third metal in the raw metal is 2 mol% or more and 93 mol% or less. According to the composite oxide catalyst produced by the production method, the oxidation start temperature of objects such as particulate matter can be lowered, similarly to the above. Moreover, according to the manufacturing method, a composite oxide catalyst containing particles having cavities inside can be easily manufactured. As a result, the oxidation start temperature of the object can be further lowered.

Various modifications can be made to the above-described composite oxide catalyst, method for producing the same, and porous composite 1.

For example, in the production of a composite oxide catalyst shown in steps S21 to S23 (that is, the production of a composite oxide catalyst by the citric acid method), the total number of moles of nitride of the raw metal in step S21 and the number of moles of citric acid are The ratio may vary. Further, the heating temperature of the mixed aqueous solution in step S22 and the firing temperature and firing time in step S23 may also be changed variously.

Furthermore, in the production of a composite oxide catalyst by the impregnation support method and the complex polymerization method, the heating temperature during precursor production and the calcination temperature and calcination time during composite oxide catalyst production may be varied in various ways. Furthermore, in the production of a composite oxide catalyst by the complex polymerization method, the ratio of the total number of moles of the nitride of the raw metal to the number of moles of citric acid in step S21 may also be varied. Note that the composite oxide catalyst may be manufactured by various manufacturing methods other than the above-mentioned citric acid method, impregnation support method, and complex polymerization method.

The porous composite 1 is not limited to the above-mentioned GPF, but is, for example, a diesel particulate filter (DPF) that collects particulate matter in exhaust gas discharged from a diesel engine. Good too. As described above, the porous composite 1 can lower the oxidation initiation temperature of the collected particulate matter, and is therefore particularly suitable not only for GPF but also for DPF. Note that the porous composite 1 may be used as various filters other than GPF and DPF. Alternatively, the porous composite 1 may be used for purposes other than filters.

The structure of the porous composite 1 may be varied in various ways. For example, the plugging portions 24 may be omitted from the base material 2. Moreover, the collection layer 3 may be provided on the inner surface of all the cells 23. Furthermore, the base material 2 does not necessarily have to have a honeycomb structure, and may have other shapes such as a simple cylindrical shape or a flat plate shape whose interior is not partitioned by partition walls.

The method for manufacturing the porous composite 1 is not limited to that illustrated in FIG. 3, and may be modified in various ways. For example, in step S12, the method of supplying the raw material slurry to the base material 2 may be changed in various ways. Further, the supply of the raw material for the collection layer 3 to the base material 2 is not limited to the filtration method using a raw material slurry, and may be performed by various methods such as a dipping method, a spray method, or a dry method. Furthermore, the drying method and drying time of the intermediate in step S13, the firing temperature and firing time of the intermediate in step S14, etc. may also be changed variously.

The composite oxide catalyst does not necessarily need to be included in the collection layer 3 of the porous composite 1, but can be used alone or in other structures to promote oxidation of particulate matter and other objects. May be used.

The configurations of the above embodiment and each modification may be combined as appropriate unless mutually contradictory.

The present invention can be applied to a composite oxide catalyst used in a filter that collects particulate matter, such as a gasoline particulate filter that collects particulate matter in exhaust gas emitted from a gasoline engine. It is.

1 Porous composite 2 Base material 3 Collection layer 22 Partition wall 23 Cell S21 to S23 Step

Claims (6)

A composite oxide catalyst that promotes the combustion of particulate matter in exhaust gas,
Containing metals include cerium as a first metal, lanthanum as a second metal, and a third metal,
The third metal is manganese or praseodymium,
The content of cerium in the contained metal is 10 mol% or more and 93 mol% or less,
The content of lanthanum in the contained metal is 3.5 mol% or more and 45 mol% or less,
When the third metal is manganese, the content of the third metal in the contained metal is 3.5 mol% or more and 45 mol% or less,
When the third metal is praseodymium, the content of the third metal in the contained metal is 10 mol%,
The content of lanthanum in the contained metal is equal to the content of the third metal in the contained metal,
The content of metals other than cerium, lanthanum and the third metal in the contained metal is 1 mol% or less,
Contains particles with a cavity inside in a SEM image with a magnification of 5000 times,
A composite oxide catalyst characterized in that the diameter of the cavity is 0.2 μm or more and 5 μm or less. The composite oxide catalyst according to claim 1,
The content of cerium in the contained metal is 80 mol% or more and 93 mol% or less,
The content of lanthanum in the contained metal is 3.5 mol% or more and 10 mol% or less,
When the third metal is manganese, the content of the third metal in the contained metal is 3.5 mol% or more and 10 mol% or less. A porous composite,
a porous base material;
a porous collection layer formed on the base material and collecting particulate matter in exhaust gas;
Equipped with
A porous composite, wherein the trapping layer contains the composite oxide catalyst according to claim 1 or 2. 4. The porous composite according to claim 3,
The base material has a honeycomb structure in which the inside is partitioned into a plurality of cells by partition walls,
A porous composite characterized in that inner surfaces of at least some of the plurality of cells are covered with the collection layer. 5. The porous composite according to claim 4,
A porous composite body characterized in that it is a filter that collects particulate matter in exhaust gas discharged from a gasoline engine or a diesel engine. A method for producing a composite oxide catalyst that promotes combustion of particulate matter in exhaust gas, the method comprising:
a step of preparing a mixed aqueous solution by mixing the nitride of the raw metal and citric acid;
heating the mixed aqueous solution to obtain a precursor;
Calcining the precursor to obtain a composite oxide catalyst;
Equipped with
The raw material metal includes cerium as a first metal, lanthanum as a second metal, and a third metal,
The third metal is manganese or praseodymium,
The content of cerium in the raw material metal is 10 mol% or more and 93 mol% or less,
The content of lanthanum in the raw material metal is 3.5 mol% or more and 45 mol% or less,
When the third metal is manganese, the content of the third metal in the raw material metal is 3.5 mol% or more and 45 mol% or less,
When the third metal is praseodymium, the content of the third metal in the raw material metal is 10 mol%,
The content of lanthanum in the raw metal is equal to the content of the third metal in the raw metal ,
The content of metals other than cerium, lanthanum and the third metal in the raw material metal is 1 mol% or less,
The composite oxide catalyst includes particles having cavities inside in a SEM image at a magnification of 5000 times,
A method for producing a composite oxide catalyst, wherein the diameter of the cavity is 0.2 μm or more and 5 μm or less.

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